Catalonia

[Gilrandir]

Were they all the richest region in the country before they gained independence?

Slovenia was. I believe Croatia came next in its economic development.

When did they gain independence and from whom?

Do I have to lecture you on history? Google it.

I'd say a region that is at the bottom end of the country it is in has better chances for improvement than one that is already at the top.

First of all, this is an arbitrary statement that needs statistical substantiation.

Second of all, since they were different in their development and now they are (more or less) prosperous countries, the starting point doesn't seem to matter.